1. Field of the Invention:
This invention relates to flexible printed circuit boards for use in electrical connection between two members movable relative to each other and, more particularly, to a method for installing the same.
2. Description of the Related Art:
The flexible printed circuit board is so thin and so sufficient in flexibility as to bend itself to any desired strong curvature and to be installed in a narrow space of the interior of the instrument with good mobility of the bent portion thereof. Hence, it has found its use in cameras at their housing and lens barrels, and other compact instruments.
To electrically connect two members movable relative to each other by means of the flexible printed circuit board, a wide variety of methods have been proposed. For example, U.S. Pat. Nos. 3,433,889 and 4,596,454 disclose that the flexible printed circuit board is curved at the center or an intermediate portion of the length to the letter "U" shaped turn, or a bellows-like waving form, thus being made to cope with the variation of the distance between the two movable members.
In FIGS. 1 through 3(B), there is shown an expanded type of the one of the above-described concepts which has the central U-turn in the flexible printed circuit board. That is, when to install the printed circuit board in the lens barrel, the space it occupies is, as shown in FIGS. 1 and 2, often taken between a fixed tube 61 whose one end is coupled with the camera body and a movable sleeve 62 for focusing or zooming.
In the case where the electrical connection from the fixed tube 61 to the movable sleeve 62, for example, for giving and receiving the zoom signals, is very long in the axial distance between the inlet and outlet for the zoom signal, the flexible printed circuit board 63 is installed in such a manner as to permit formation of two letter U-shaped bent portions 64 and 65 as shown in FIG. 2. And, these bent portions 64 and 65 are arranged to change their axial positions in accompaniment with movement of the sleeve 62.
In such a manner, according to the prior art, for the case in which the ends of the printed circuit board are connected to the respective points in position spaced far away from each other, the required number of bent portions of letter "U" shape to be formed midway therebetween was two as indicated at 64 and 65 when it was installed in the narrow space. For this reason, the space between the fixed tube 61 and the movable sleeve 62 (the radial length of the gap) had to be taken, in principle, at as high a value as the sum of the diameters A and B of the U-shaped bent portions 64 and 65 respectively. This led to a problem that the lens barrel was unavoidably of large diameter. If the height of that space was decreased, it would result that, as shown in FIG. 3(B), one of the bent portions, in this instance, 64, is caused to come into contact with the substrate of the board 63. In this case, therefore, as the repetition of operation of the movable sleeve 62 increased, the frictionally contacting areas of the upper surface of the bent portion 64 and the lower surface of the board 63 eventually worn out with a damage of the cover layer. Also, because the movement of the movable sleeve 62 occasionally gave stress to the board 63, the bent portions 64 and 65 were hindered from smoothly changing their positions. Again, if such a stress was suddenly released, the board 63 would take an abnormal form. Thus, there was a high possibility of damaging the electrical fidelity of the printed circuit board. And, in order to insure that the printed circuit board smoothly moved itself despite the presence of the two bent portions of letter "U" shape, the diameter of the movable sleeve 62 tended to increase.
Next, using FIG. 4, another problem of the conventional example is described. For the electrical connection from a fixed tube 51 to a movable sleeve 52, for example, the electrical power supply line and the input and output lines for the zoom signal, motor control signal and lens mode selection signal, a stripe-shaped flexible printed circuit board 53 is set up in such a way as to form a bent portion 54 of letter "U" shape.
To allow the bent portion 54 to change its axial position under such a condition that a part of the board 53 which is on the upper side of the bent portion 54 is always in contact, over the entire width, with the inner surface of the movable sleeve 52, the movable sleeve 52 is provided with an axial shallow groove 55 of flat bottom.
With this, when the movable sleeve 52 moves axially, the bent portion 54 can sweep along the groove 55 without causing its side edges to be pressed against the otherwise curved inner surface of the sleeve 52. Hence, no unduly large stress is applied to the circuit pattern of the board 53 at the center of the width thereof. Thus, the board can change in position without damaging the electrical fidelity.
However, formation of such an axial groove 55 by machining in the inner surface of the sleeve 52 under the condition that its bottom was maintained sufficiently flat over the entire length was far more difficult than when it was formed on the outer surface of the tube 51.
Particularly when the axial length of the groove 55 got longer, that difficulty was rapidly increased regardless of whether to employ the key seat milling machine or the plastics molding means. In the latter case, the structure of construction of the mold became very complicated. Also, since the thickness of the sleeve below the groove is very short compared with the other part, when the parts are made up of plastics by molding means, such a partial disconformity of the thickness is not favorable from the precision accuracy standpoint and in an aesthetic sense.